The objective of this application is to elucidate in more detail, the mechanism of action of these drugs of abuse, especially in how they interact with central cholinergic systems. Recent evidence from our laboratory has shown that the intraventricular injection of acetylcholine produces antinociception and interacts with opiate receptors similar to those produced by parenterally administered morphine. We have now shown that acetylcholine and other cholinergic compounds also block the stereospecific binding of narcotics to the opiate receptors. Acetylcholine and morphine have been shown to increase the ratio of cyclic GMP to cyclic AMP and cyclic AMP has been shown to antagonize morphine induced antinociception. Therefore, we investigated the effects of cyclic GMP as an antinociceptive agent. Cyclic GMP but not most other guanosine nucleotides have antinociceptive activity. Calcium antagonizes the antinociceptive effect of morphine and acetylcholine but potentiates the activity of cyclic GMP. We hypothesize that acetylcholine and morphine produce antinociception by interacting with their respective receptors which are in close proximity on the cellular membrane and that cyclic GMP produces its effects intracellularly. Calcium anatagonizes the antinociceptive effects of acetylcholine and morphine by overcoming their inhibitory effect on calcium exchange in the neuron and it potentiates cyclic GMP at intraneuronal sites. Morphine and acetylcholine cause a release of endogenous opiate ligands from brain.